1. Field of the Invention
This invention relates generally to the digital data processing art and more particularly to a new and improved method and apparatus for encoding and recovering binary digital data. The invention is particularly applicable to digital data communication systems and to magnetic storage and retrieval systems, and will be described herein with reference to the latter.
2. Description of the Prior Art
In the course of development of binary data magnetic storage and retrieval systems, it has been of primary interest to increase the system data capacity by packing as much data as possible into a given time interval or length of the recording medium such as a disk or tape. This objective is met by encoding the binary data so as to place or store signal changes or transitions representative of the respective ones and zeros of the binary data as close together as is practical. Various constraints intrinsic to such systems impose a limitation, however, on what is in fact practical, insofar as data packing density is concerned, with respect to accurate recording and reproduction of the data. One such constraint is a phenomenon commonly referred to as bit shift which occurs in the course of reproducing the binary data from the encoded signal recorded on the storage medium. It is characterized by a shifting of the reproduced signal transitions from their nominal locations and is caused by the close proximity of crowding of adjacent transitions recorded on the storage medium. More specifically, bit shifting occurs as a consequence of the interference or interaction of each reproduced signal transition with adjacent reproduced signal transitions when reading the recorded signals from the storage medium. The amount of shift which occurs for each reproduced signal transition is determined by the packing density and the degree of asymmetrical disposition of the transitions adjacent both sides of each reproduced signal transition, with the amount of shift being proportionally greater in accordance with increased packing density and asymmetry of the respective signals.
Bit shift is of considerable concern because it directly relates to the ability to accurately reproduce the binary data as will become apparent from the following comments. When data is to be recorded it is encoded, as previously mentioned and as will be explained subsequently in greater detail, and then applied to the storage medium on a clocked basis so that each signal transition is recorded in a prescribed interval or segment of the storage medium. Recording on a predetermined time basis is essential to enable detection of the respective one and zero data bits when reading from the storage medium for the purpose of reproducing the binary data stream. Typically, a gated oscillator or preferably a phase locked oscillator is employed to create a time oriented window for recovering the binary data from the reproduced signal transitions. The phase locked oscillator, for example, usually functions in a manner, as is well known to those skilled in the art, such that it runs at a nominal frequency which is a selected harmonic of the frequency corresponding to the fundamental period of the encoded data signal and thereby produces a gating window signal associated with each reproduced signal transition for recovering the binary data from the encoded data signal. At this point it should be understood that the recovery window has associated with it a feature commonly referred to as timing tolerance. It will be appreciated that when the signal transitions are packed closer together, the recovery window must be narrowed to preclude detection of a reproduced signal transition at a nonassociated window. Naturally, as the recovery window is narrowed, the amount of bit shift which can be tolerated is reduced proportionally. A phase comparator included in the phase locked oscillator serves to compare the phase of the reproduced signal transitions read from the storage medium with a signal supplied from the phase locked oscillator to produce a signal for controlling the oscillator so as to cause it to track the reproduced signal transitions. A filter circuit of the phase locked oscillator functions to enable the oscillator to track average time locations of the reproduced signal transitions while remaining insensitive to instantaneous variations thereof. In this way, the recovery window is maintained in general alignment with the reproduced signal transitions. In the case of any abrupt bit shift in excess of a predetermined amount, however, the reproduced signal transition will be positioned outside its recovery window with resultant failure of detection and erroneous data recovery.
From the foregoing comments it will be appreciated that bit shift must be reduced to enhance data recovery and that reduction of bit shift in turn is dependent on avoidance of inordinate crowding of adjacent transitions of the encoded data signal. To satisfy such criteria and others which will be discussed subsequently, various encoding techniques have been divised in the development of the art. Some of the desired characteristics of a suitable encoding technique will be discussed briefly at this point and explained more fully hereinafter in connection with the detailed description of the instant invention and selected prior art codes illustrated in the appended figures. One of the desired characteristics, of course, is that the encoding be such as to avoid undue bit shift. This is achieved by providing sufficient spacing between successive signal transitions recorded on the storage medium, but must not be done at the expense of reducing the recording density. Another desired characteristic of any encoding technique is that it avoid such large spacing between recorded signal transitions as would preclude the ability to achieve self-clocking during data recovery. Self-clocking is a feature whereby the encoded signal recorded on the storage medium and the related readout or reproduced signals possess such qualities as to provide the required control of the phase locked oscillator for data recovery as previously discussed. In the absence of a self-clocking capability, a separate clock channel must be provided on the recording medium and this is undesired, among other reasons, for the reason that it requires maintaining alignment of the read/write head of the clock channel relative to the heads associated with the data channels. The requirement, on the one hand, of sufficient minimum spacing between successive signal transitions so as to preclude undue bit shift and the requirement, on the other hand, for a limited maximum spacing between successive transitions so as to achieve self-clocking is essentially equivalent to the criterion that the number of recorded transitions per data bit be minimized or conversely that the number of data bits represented by each recorded transition be maximized.
The various encoding techniques commonly used in the present state of the art are generally deficient in one respect or another relative to the above indicated characteristics. So-called NRZ or NRZI codes, for instance, are characterized, in the case of a succession of several one or zero bits, by long intervals between recorded signal transitions thereby precluding self-clocking. Frequency modulation (FM) and phase modulation (PM) codes, on the other hand, while providing a self-clocking capability, are characterized by close spacing of the recorded signal transitions and thus limited in data packing density and timing tolerance as required to avoid undue bit shift and assure accurate data recovery. The close transition spacing of the FM and PM encoding techniques occurs because of the periodic insertion of clock transitions into the stream of data transitions for the express purpose of achieving self-clocking and thus these codes are degraded with respect to the desired criterion of minimizing the number of recorded transitions per data bit.
A more recently developed code known as modified frequency modulation (MFM) overcomes the limitations of the FM, PM and NRZ type codes to some extent and has in fact been in popular use for the past several years because of its self-clocking capability and the provision of substantially twice the packing density of the FM and PM codes without aggravating the bit shift problem or reducing timing tolerance. The MFM encoding technique does not employ additional clock transitions but instead uses the data transitions for clocking purposes and thus provides enhancement with respect to the criterion of minimizing the number of recorded transitions per data bit. Nevertheless, the binary data packing density which can be achieved with MFM code is limited by the minimum spacing which it provides between the successive signal transitions recorded on the storage medium. This limitation of the MFM code relative to the instant invention will be understood more fully from a reading of the subsequent detailed description of the presently preferred embodiment of the invention.